The exquisite timing and validating effect of the New Hampshire Union Leader’s endorsement presents Gingrich with the biggest boost yet to his resurgent campaign — a conservative stamp of approval at a pivotal moment.

Not only does the former House speaker now have a powerful political voice in his corner as he establishes himself in polls as Mitt Romney’s chief rival, he has it at the very moment his immigration stance is receiving fresh scrutiny from party activists. Now the question is whether the wave of momentum, cresting just weeks before votes are cast, will be significant enough for Gingrich to sweep away unease on issues where he isn’t in alignment with the base.

While the Union Leader’s imprimatur isn’t consequential enough on its own to winnow the field, it’s an important milepost for anyone who hopes to emerge as the conservative alternative to Romney.

“Major credibility for Newt when he needs it most,” said New Hampshire-based GOP strategist Mike Dennehy, a key operative in John McCain’s two Granite State wins, of the newspaper’s backing. “[It] helps raise money, helps on messaging to justify momentum and will likely help in defending some issues like immigration.”

Patrick Griffin, another unaligned New Hampshire Republican consultant, noted the paper doesn’t hold back when it comes to advancing the interests of its chosen candidate.

“If and when Mitt attacks Newt, they will protect him,” said Griffin. “They smell blood and on top of their interest in asserting the paper’s ideology, they see this as a chance to deny Romney a coast to the nomination.”

While the endorsement itself made no mention of Romney and allowed that Gingrich was imperfect, Union Leader publisher Joe McQuaid suggested Sunday that they would reprise their tradition of near-daily editorials that reiterate full-throated support for their favored candidate and equally robust opposition toward his challengers.

“I think we will be sticking with our traditional approach,” McQuaid wrote in an email.

Merely by delivering an endorsement when they did, the paper helped temporarily blunt the immigration issue that is complicating Gingrich’s prospects. The combination of the Thanksgiving hiatus and McQuaid’s much-buzzed-about nod offers a temporary reprieve to Gingrich from the media attention he sparked by outlining what he called his “humane” immigration position at last week’s debate.

Gingrich raised significant doubts with those remarks, which cut too close to a comment Rick Perry all but disqualified himself with among conservatives at an earlier debate. How the former speaker handles the issue now could be determinative. If he can convince GOP activists in the coming days that his immigration plan does not amount to amnesty, it could resolve the lingering question of the Republican contest: who will give Romney a run.

If he can’t, he’s likely to return to the pack and the uncertainty that has marked the campaign to date will reassert itself, leaving the former Massachusetts governor with a fractured conservative opposition going into the homestretch before voting begins.

As important a jolt as the Union Leader has sent into the New Hampshire and national GOP conversation, it’s in Iowa where Gingrich will be most acutely tested.

That’s where immigration burns hotter than most other issues at the grass roots, where the former speaker is already facing attacks from some of his Republican rivals over the issue and where votes are first cast just over a month from now.

Gingrich returns to Iowa this week to speak at a county GOP fundraising dinner. Yet it’s unclear what he’ll do to push back against critics of his proposal to grant longtime illegal immigrants with a “red card” that wouldn’t confer citizenship but would allow them to stay in the country. Appearing before large crowds in Florida over the weekend, Gingrich proposed “a jury system for local communities” that would decide which illegals are allowed to remain in America.

With a shoestring campaign organization, it could be difficult for the former House speaker to effectively explain an issue on which anything less than a hard-line is viewed with suspicion by the GOP base. With an influx of money owing to his recent surge in the polls, he’s been hiring staff and opening offices. But it’s unclear whether he’s prepared to quickly craft direct mail or TV and radio ads on immigration.

Clearly, though, he must make an attempt to do something in response — and do so immediately.

“I count Gingrich as one of those who peaked and is now descending,” said western Iowa GOP activist Ann Trimble-Ray, citing the boom-and-bust pattern that has marked the Republican race to date. “Immigration [is a] third rail for Iowans and Newt landed right on it.”

His rivals know it.

For the conservatives trailing Gingrich who need a strong showing in Iowa — Rick Santorum and Michele Bachmann — immigration presents their best and potentially last chance to knock out the Republican now standing between them and a clean shot at Romney. And for Romney himself, immigration is a potent issue that he could use to get to the right of Gingrich and raise doubts among the party’s base about just how conservative the would-be conservative alternative is to the establishment favorite. Then there is Ron Paul, no Gingrich fan, who appears to have a ceiling on how much mainstream Republican support he can draw but also has the money on hand to do serious damage to his rivals on the airwaves.

Santorum communications director Hogan Gidley indicated in an email that they’d be confronting Gingrich over more than just his immigration stance.

“While the press has been dying to write Romney vs. anti-Romney — Newt’s record clearly shows he’s is not the anti-Romney, he’s Romney-redo,” wrote Gidley. “Whether it’s amnesty, the individual health care mandate, cap and tax or TARP — Speaker Gingrich has taken the same positions as Romney.”

In Iowa over the weekend, it was Bachmann who hammered at Gingrich.

“He has a long history of supporting amnesty, and that’s not something that people in Iowa are supporting,” Bachmann said at a book-signing stop Saturday in West Des Moines. Her staff handed out to reporters copies of a 2004 letter to The Wall Street Journal promoting comprehensive immigration reform that the former House speaker signed along with other Republicans.

Also arriving over the weekend in Iowa were Romney’s first mail pieces, one of which touted him as “the strongest Republican to beat Barack Obama and end illegal immigration.”

While most of their for-public-consumption attacks have been against Obama, Romney’s campaign has also shown a willingness to take on its GOP rivals: The candidate himself took Gingrich to task on immigration in a Des Moines visit last Wednesday. Mail and phone calls criticizing the former House speaker on it could be put together in short order.

But going negative in multicandidate primaries is always a risk for the candidate on the offensive — it could inadvertently help a different candidate — and Republican voters seem to be especially sensitive about intraparty negativity this cycle as they weigh who will take on Obama.

“Herman Cain has been consistently positive about his opponents,” noted Steve Grubbs, Cain’s Iowa director. “I have no indication that’s going to change.”

But when asked, Grubbs did allow: “Newt’s position on immigration is not consistent with the views of Iowans.”